Only have eyes for/have eyes only for somebody
Definition 1 of 1
Pro Tip 1/3Romantic Focus
Emphasizes a strong romantic interest, suggesting that the person is ignoring others. 
Despite the many eligible bachelors, she only has eyes for John.
Pro Tip 2/3Exclusivity
Implies that no one else is being considered in a romantic context. 
He's asked out many women, but he only has eyes for his childhood sweetheart.
Pro Tip 3/3Literal Usage
Not actually about sight, but rather attention or affection towards someone. 
Even in a crowded room, it's obvious that she only has eyes for her partner.